Electronic switches represent a very great part of the advance in electronics as a whole in the last few decades, replacing metallic contacts in virtually all fast applications. All Pulse Width Modulation [PWM] power circuitry depends on fast electronic switching. At low to moderate supply voltages, typically below 50V, MOSFETs are extremely effective switches. Toward the high end of this range, however, MOSFET based high side switch drivers are problematic and expensive. At 100V and above, MOSFETs usually require 10V of drive voltage, and the conductive channel must be limited in conductivity in order to meet breakdown voltage ratings. Both of these restrictions limit the transconductance [Gm] of the device. A simple junction NPN transistor requires less than one volt to be fully turned on, and by means of beta (carrier multiplication) can achieve very high current densities. The result is a dramatically higher Gm for a device of the same active die area.
Unfortunately, prior art driving means utilizing junction transistors have typically been arranged in a common emitter mode, often requiring separate high side or low side driver stages, and are difficult to drive directly from ordinary low voltage integrated circuits.